Honestly I wouldn't go by marathons.
I was reading today that a newsreporter in some sports magazine or another who was fairly athletic, but by no means specialized in any area ran a 26 mile marathon only 4 minutes over what Oprah did.
Her, with 4 months of training for the marathon, and him with none.
Personally, playing full court basketball & track most my life - I know running for endurance won't do much to overall strength and body shape for guys. Upper body size is just too crucial.
1) What are the best methods in getting in better shape in terms of your endurance? Can you do that without losing significant mass?
Endurance is overrated. Skinny Nigerian olympic competitors have endurance, but at the cost of an attractive body shape. Endurance is only good when combined with comparable increases in strength.
2) How can you measure yourself in order to know whether you have "poor" endurance as opposed to "good" endurance or "excellent" endurance.
Very hard question to answer. On what subject? If its pull-ups I would say 5 and below is poor endurance, 6-10 is good endurance, and above is excellent. For the mile I would say 10 minutes and above is poor endurance, 6-9 is good endurance, and below is excellent. BUT, a body builder will likely succeed more at the pull-ups while a skinny person might excel at the mile.
3) Do the kind of exercizes the military do to make a solider stronger and endure more stress is an ideal fit?
I don't know military, but I know one of the major components of exercise a lot of people miss is lower abs & hip flexors.
Crunches are "ok" but, leg-ups are the bee's knees. Basically the point of your body you would fold in half with. These are hit by leaning into turns while jetskiing, wrestling, swimming, etc. Activities, a lot of people may not participate in on a regular basis.
A bodyweight exercise that can simulate this are legups where you're holding the table leg's ankles above your head, with your back on the ground. Your legs, lying flat, are raised toes stretched straight. Then, with toes pointed, raise your legs so your toes touch the tabletop's vertical (typically a bit above the table-top itself, or in the case where you have a partner's legs to grab onto, into their solar plexus.) When you move back down to the ground don't go below 6 inches, then repeat another rep. It's also killer on your intestinal/kidney/midsection area which plays a large part in endurance. Try to get to 60.
I would post a pic of the particular leg-ups I'm referring to, but google isn't playing nice.